Known in the art are pneumatic percussive devices having valve and spool air distribution arrangements and pneumatic devices in which air distribution is effected by the hammer piston. All such devices are characterized by the provision of a system of passages made either in the casing walls or in the hammer piston, which are necessary for controlling operation of the spool or valve and also for supplying compressed air to working chambers of the device and for discharging exhaust air from these chambers. The provision of such passages results, on the one hand, in a decrease in the net area of the hammer piston which, in turn, lowers the specific impact power and, on the other hand, complicates the hammer piston and casing thus bringing about superfluos stress concentrations so as to substantially reduce service life of these parts. This is true to the largest extent for underground tools such as pneumatic moles in which the diameter of the casing, hence of the hammer piston, is limited by the diameter of the hole, and the impact loads are taken up not only by the hammer piston, but also by the casing which functions as a working member as well.
Known in the art is a pneumatic percussive device (DE, C, 1132067), comprising a pile hammer lowered into a borehole and an independent air distribution arrangement installed on the ground level. The pile hammer is in the form of a simple impact work consisting of a tubular casing closed at both ends and a hammer piston mounted therein for axial movement. The hammer piston divides the interior space of the tubular casing into two chambers communicating with each other either through a throttling passage, or through a passage having a check valve, or by means of both. At least one of these chambers, which is referred to as the control chamber, communicates through a hose with the air distribution arrangement provided on the ground level.
The air distribution arrangement is generally in the form of an oscillating system consisting of a spool valve box and an actuator provided therein and made in the form of a spool or a valve adapted to perform oscillations either automatically or positively under the action of a drive mechanism, e.g. a cam drive. The self-oscillating spool is connected by means of levers and pivot joints to a pendulum having an adjustable weight.
For putting the pile hammer in operation, the actuator of the air distribution arrangement is automatically or positively driven to perform oscillations. During oscillations of the spool the hose connecting the controlled chamber of the pile hammer to the air distribution arrangement alternately communicates with a compressed air source and with the environment depending on position of the spool, whereby the controlled chamber of the pile hammer also alternately communicates with the compressed air source and with the environment. Consequently, pulsating pressure is built up in the controlled chamber. As both chambers of the pile hammer communicate with each other through the throttling passage or through the passage incorporating a check valve, rather than through a free passage, pressure in these chambers is always different. Under the action of the pressure difference in the chambers, the hammer piston performs reciprocations during which it imparts blows either to a working implement or to the casing-in the opposite direction. The desired direction of blows is ensured by a preset combination of parameters of the air distribution arrangement chosen by way of experiments.
In certain embodiments of the pile hammer there are no passages in the hammer piston and casing altogether. This makes the abovedescribed device advantageous over prior art pneumatic percussive devices having a spool or valve air distribution arrangements that cannot be implemented without a system of passages which are required for controlling the spool or valve and for discharging waste air from the chambers and admitting compressed air to the chambers.
The provision of a hose connecting the controlled chamber to the air distribution arrangement which is located at a substantial distance from the pile hammer results in an increase in the "dead volume" of this chamber by the amount of the volume of the interior space of the hose. At the same time, an increase in the "dead volume" of the chamber is known to result in an additional unproductive consumption of compressed air, hence in a lower efficiency. In addition, a substantial length of the hose limits the rate of pressure pulses effectively transmitted to the chamber, i.e. limits impact power of the pile hammer. In an ideal case, the rate of pressure pulses effectively transmitted through the hose per unit of time is determined by the formula: ##EQU1## wherein .omega. is the rate of pulses;
L is the hose length; PA1 V is the velocity of sound in the air.
In real life devices, the rate of effectively transmitted pulses is still lower.
Known in the art is a pneumatic percussive device (SU, A, 261319), comprising a casing and a hammer piston mounted in the casting for movement. The hammer piston divides the interior space of the casing into two chambers.
A working implement is incorporated in the front end part of the casing. A massive balancing piston is provided in the rear end part of the casing, which is in the form of a spool adapted to perform self-oscillatory movement when compressed air is supplied to the device. A longitudinal passage provided in the spool permanently communicates with a controlled chamber on one side and is communicatable with either a source of compressed and or the environment on the other side depending on position of the spool. Owing to the fact that the independent air distribution arrangement is incorporated in the casing of the percussive device, there is no need to use a hose for communication of the controlled chamber with the air distribution arrangement as was the case in the prior art pneumatic percussive device of DE, C, 1132067.
When compressed air is supplied to this device, the spool provided in the rear end part of the casing performs self-oscillations with respect to the casing. Depending on position of the spool during its oscillatory movement, the controlled chamber alternately communicates through the longitudinal passage of the spool with the compressed air source and with the environment. Therefore, the balancing piston which is made in the form of the spool functions not only as a balancing inertia member but also as an independent air distribution device establishing communication alternately between the controlled chamber of the device and the compressed air source and the environment. Under the action of pulsating pressure in the controlled chamber and air pressure in the other chamber, the hammer piston performs reciprocations during which it imparts blows to the working implement.
However, as the balancing piston functions as a balancing member as well, it has substantial dimensions and mass as well as the amplitude of oscillations so that the size and mass of the device as a whole also increase without bringing about any increase in the impact power. Consequently, the specific impact power of the device is lowered. Attempts made to reduce mass and size of this prior art device by way of rational choice of dimensions, mass and swing of oscillations of the balancing piston and by lowering amplitude of oscillations of the balancing piston by means of limiting abutments failed. Thus reducing mass of the balancing piston to lower mass of the device as a whole inevitably result in the increase in amplitude of its oscillations so that length of the casing increases and the reduction of mass of the device as a whole is not achieved because of an increase in mass of the casing. Reducing amplitude of oscillations of the balancing piston by means of the limiting abutments as is the case in known pneumatic percussive devices with valve air distribution arrangements in which the valve remains stationary alternately in one and other position is also impossible for two reasons. Firstly, the balancing piston cannot be stopped if one wants it to perform its function. Secondly, this piston being a self-oscillating member, it cannot remain stationary after its engagement with the abutment since the very principle of its self-oscillation movement involves the development of a rebound force under the action of which the balancing piston is instantly reversed after its stoppage. As a result, frequency of oscillatory movement of the balancing piston is only determined by a very short time of its shift between the two abutments and it will become too high as to rule out normal operation of the device.